Rapid growth in population, industries, infrastructure development in the urban area tremendously increased the traffic volume which resulted the traffic congestion on the roads led to shortage of road space at ground level, therefore, there is need to create extra space above or below the ground level to meet out this demand. Construction of elevated roads/railways disturb the traffic system, however, the underground structures like multilevel roads or underpasses, road tunnels, metro systems do not disturb the surrounding. In this regards the other scopes of underground structures like; Malls, multilevel basements, water supply, flood water storage tunnels, sewers, cable tunnels, substations, air raid shelters, and storage facilities, etc tremendously increased in the city. To meet out the current traffic and other demands, civil engineers have been using valuable underground space beneath the urban areas. All these underground structures involve huge construction cost time and manpower not only this, these structures also required special construction skill. Moreover, all these structures associated with foundations. Therefore, the stability of these structures is at most important. The stability of these structure is mainly depends upon the foundation soil and vertical stability of side walls. In nature soil is generally exist in heterogeneous state, it is not necessary that all the time soil condition may suit the structural requirements. The inadequate stability of slope can be improved by suitable ground improvement techniques. There are many methods available for ground improvement. Soil Nailing Technique has proved a safe and economical solution (10% to 30%) if we compare with the other method of stabilisations.
In general, the requirement of shallow depth tunnel is more because of usability, availability of land and project costs. It has been analysed and found that stability of shallow depth tunnel are very less as compared to deep tunnels. The shallow depth tunnels have been constructed using cut and cover techniques, which have often proved highly disruptive requiring road closures and property demolition. At shallow depth the natural arching properties of ground do not develop. With the advancement of civil engineering in globe, the pre-cast technology had proved a time and cost saving technique for the construction of civil engineering projects. In-pre cast technology, the project is usually constructed in steps followed part to the whole. Therefore, the scope of underground construction using pre-cast panel is gaining popularity. Therefore, our research emphasised on stepwise stabilisation of soil inside and outside of precast box to be pushed through the jacking technique into the existing soil masses.
References may be made to U.S. Pat. No. 4,009,579, wherein Delbert M. Patzner et. al provide a method for constructing tunnels and underpasses quickly and inexpensively, with conventional readily available equipment, without disrupting existing constructions and without interrupting or delaying service thereon. The method comprises the inserting a plurality of longitudinal support members side by side through the ground beneath the existing structure followed by excavating a longitudinal increment of the ground beneath the support members. Thereby, installing tunnel forming precast sections beneath the support members in place of each longitudinal increment of excavated ground to support which the support members, repeat the excavation and placement of tunnel forming section till the full length of tunnel is covered.
References may be made to patent U.S. Pat. No. 4,139,320 wherein, a process for excavating and constructing a tunnel with the help of an excavating device excavator equipped with a screw conveyor has been provided. This process is directed to excavating and constructing a tunnel under a railway or a road on a bank or on level land in the direction transverse to the railway or road. In this process, pits are dug on both sides (entrance and exit end). In this respect, a hollow casing unit of a box shape is coupled to the rear end of the excavator equipped with a screw conveyor. As the excavator advances or digs forward a given distance, another casing unit is in turn coupled to the rear end of the preceding casing unit, and then such a step is repeated, until the excavator goes out of the wall of another pit. The sand and soil inside the outer wall of the tunnel are excavated and removed, after which reinforcing steel bars and a mold are placed along the inner surface of the wall of the tunnel. Concrete is then poured into the hollow casing units themselves as well as between the mold and the wall. Thus, the hollow casing units form an integral part of the wall of a tunnel, as an outer wall.
References may be made to patent U.S. Pat. No. 4,405,260 wherein a method of constructing underpass across railway and highway without affecting normal traffic thereof, the steps of excavating a traction ditch on one side of the road foundation and a launching ditch on the other; building a traction wall with traction holes therein against the road foundation in the traction ditch; and sequentially tracing a precast box culvert one after another through perforating, anchoring and jack driving according to the construction line until a predetermined configuration is completed thereat. Subsequently, build pier foundations, supports, and a bridging beam; arrange shell pipes; place PC steel reinforcements; and, after a certain curing period, perform pre-stress operations in the precast box culverts of the structure and grout cement mortar therein. Finally, excavate the earth volume under the structure and finish the road surface of the underpass for opening to traffic.
The previous patents/intentioned revealed that, soil stabilisation part inside the tunnel boundary has not been covered by any of the investigators. All the above said methods used for generalized soil conditions. The present invention is fruitfully worked for all collapsible soil/generalized soil conditions and any kind of loading conditions. The stabilisation of soil using ‘Soil Nailing Technique’ will be viable solution for such kind of underpass constructions.
In this patent application, step wise stabilisation of soil slope inside and outside of the tunnel and construction of underpass is explained in a simplified way irrespective of soil type/conditions.
To carry out this task, detailed field and laboratory investigations were carried out and all the relevant data pertaining to the project was collected. Based on our previous experience of handling projects of underground construction and the problems of slope stability in hilly terrain in landslides prone areas, it was decided to adopt ‘Soil Nailing Technique’ for the stabilisation of vertical cut slope in sandy strata to facilitate the box pushing through sand, while maintaining the movement of train without any interruption.
Soil Nailing is a relatively new construction technique used in Europe and America but very little work in this regard is carried out in India. Soil nailing consists of reinforcing the soil mass by introducing a series of thin elements called Nails to resist tension, bending and shear stresses. The reinforcing elements are made of steel round bars called as Nails. Nails are installed sub-horizontally or horizontally into the soil mass in pre-bored holes, which are grouted along their full length to form “Grouted Nails” or simply driven into the ground, called as “Driven Nails”. The nails or metallic reinforcement, which are installed horizontally into the soil mass improve the shear strength and resist bending and tensile stresses developed in soils under loading. This technique is generally recommended to stabilise cut slopes, which are cohesive in nature and under static overburden pressure. However, under this project, the technique is conceptualised for stabilising pure sandy soil of collapsible nature under heavy dynamic loading. The concept was initially tested in a small scale laboratory model studies. Based on the observations a design was developed for a large scale live project with heavy dynamic loads with the help of nails and supportive plates. The bending and shear stresses were checked at different locations in the entire soil mass to prevent failure due to shear and surface erosion.
Though it was quite difficult to replicate the field conditions in the model test, nevertheless, the model studies provided a great insight to understand the behaviour of mass movement of sandy soil under heavy and dynamic loads with and without soil nailing. On the basis of the model studies, strategy for design and construction methodology of the project was formulated. The technique helped in successfully pushing the three boxes and creating an underpass in a record period of time without any kind of problem and this has resulted to open the bye-pass road much before the commencement of the commonwealth games.
This technique was tried first time in the world for such a kind of project in zero tolerance zones.